Signal receiver circuits of the above mentioned general type are used, for example, in transponders for contactless identification systems such as radio frequency identification (RFID) systems, and for remote sensors, in order to receive and decode a coded signal that is transmitted from a base station or a reader device, and then to transform the decoded signal into a digital signal. For this purpose, such circuits typically include a rectifier that rectifies the signal received by an antenna of the transponder. A so-called signal capacitor is connected at the output of the rectifier. The signal voltage that represents the presently transmitted code prevails on the signal capacitor and is evaluated by an evaluating circuit connected thereto. In order that the signal voltage will follow a change or variation of the input voltage of the rectifier as quickly as possible, the signal capacitor has a defined discharge current applied thereto from a discharge current source or a discharge current sink. The discharge current has a magnitude adapted to the capacitance of the signal capacitor, while the capacitance is selected so that short time constants can be achieved and simultaneously a UHF decoupling is ensured.
Such a circuit arrangement with a discharge current sink, which comprises a current mirror circuit, is disclosed in the U.S. Pat. No. 5,889,489 (Friedman et al.), for example. The entire disclosure of U.S. Pat. No. 5,889,489 is incorporated herein by reference. In the known arrangement, the current mirror circuit is embodied as a so-called simple current mirror consisting of two n-FET transistors, in which a rated or desired current value in the form of a reference current, through one of the transistors, determines the current strength of the other transistor, that is to say the discharge current. Due to the characteristics of the MOS transistors used in such conventional current mirrors, the current strength is reduced sharply for signal voltages that are smaller than the transistor saturation voltage. Moreover, even for signal voltages above the transistor saturation voltage, there still exists a dependence between the discharge current and the signal voltage.
Current sources in the form of current mirror circuits, which have a small dependence between the current and the voltage, are utilized in operational amplifiers, for example. A so-called cascode current mirror circuit with a large output range, or so-called “wide-swing cascode current mirror” for an operational amplifier, which considerably reduces the dependence between the current and the voltage, is described in the article by D. A. Johns and K. Martin “Analog Integrated Circuit Design”, pages 256 to 259, ISBN:0-471-14448-7.